The present invention relates to an assembly unit that can be attached to a base structure, the assembly unit comprising an assembly part provided with at least one through opening and at least one fastener consisting of a radially expanded head and a shaft that extends through the opening and is held captive with axial mobility therein, wherein this shaft has a fastening section on the end that is removed from the head. Assembly units of this type are known from the German patent documents DE 196 54 848 C2 and DE 296 20 397 U1.
During the assembly, an assembly part of this type is moved toward the base structure, wherein the fastening sections, which can be supplied from above during an assembly, will project from the bottom of the through opening for reasons of gravity. If the assembly part is supplied essentially parallel to the center longitudinal axis of the fasteners, then the fasteners and/or their fastening sections are pushed back into the through openings once they hit the assembly surface of the base structure. In some assembly situations, however, the fastening sections that can still project from the through openings can cause problems, for example when they get snagged on edges or projections of the base structure. With a base structure having two assembly surfaces that are inclined toward each other and one assembly part with correspondingly angled counter surfaces, the fasteners are admitted with a force arriving at an angle to their longitudinal extension when they impact with the assembly surfaces, thereby obstructing or even preventing a pushing back into the through opening. This problem also occurs if the assembly unit is supplied to the base structure from a direction that extends at an angle to or is parallel at a short distance to its assembly surface, for example for space reasons.
The assembly part frequently consists of a relatively flexible material as compared to the base structure, e.g. a plastic material. As a result of using an elastic material for the assembly part, the pre-tensioning force required for a secure connection cannot be achieved with the fastener because, in the fully assembled state, the fastener only supports itself on the assembly part, which would consequently be compressed and deformed in an impermissible manner.